1. Field of the Invention
This invention relates to the so-called light recording medium which reads information given as a physical deformation on a recording layer.
2. Description of the Prior Art
What have heretofore been employed as media for recording information are magnetic recording media, but they have several disadvantages, for example, the information recording density is limited, there is also a limitation on improvement of noise characteristics, the medium is not free from abrasion since it is used in the state where the medium is in intimate contact with a head, and so forth. For that reason, light recording media have recently come to attract attention as the substitute for the magnetic recording media.
Among the light recording media, the so-called head mode recording has a fundamental constitution of a recording layer to which a physical deformation is imparted by irradiated light and a substrate for supporting said recording layer. On recording information coherent light such as laser light is irradiated to melt or burn the medium to provide a small hole thereon, and recording of information is effected by the presence or absence of said small hole.
As materials for the recording layers used in the prior art heat mode recording, tellurium, tellurium-selenium-arsenic compounds, mixtures of nitrocellulose and light absorbers, etc. are known. In the case of each of the above-described materials, the record is permanent and hence erasing and rewriting of the record are not possible, and as media which permit erasing and rewriting of records, there have been proposed in Japanese Patent Application No. 56-51160 styrene-based low polymers (e.g. styrene oligomer) and organic polymers such as nylon, ABS resins etc. which effect recording by a thermal deformation of a recording layer.
Of such constituent materials from the recording layers, those utilizing organic dyes as light absorbers have advantages that they are non-toxious to human beings and that they permit enlargement of the surface area of a recording layer, such advantages having not been achieved with the metal-based recording layers, and further their low cost also makes them easily usable as recording materials from a costwise aspect. On the other hand, each organic dye, while having such merits, has a nature to manifest good absorptivity only against certain light wavelengths, and therefore on use thereof, it is necessary and hence troublesome to change the kind of the contained dye according to the kind of the laser light source. In other words, since the wavelength of a helium-neon laser is 633 nm, an argon laser 515 nm and 488 nm, a helium-cadmium laser 442 nm and 325 nm, and a semiconductor laser 830 nm, 780 nm and 750 nm, it is necessary to constitute a recording layer according to the laser light source used. Especially with the semiconductor laser, since the current situation is such that their wavelengths are being made shorter and shorter with the progress of researches and developments, and hence it is difficult to establish a certain wavelength, it poses a grave problem on the constitution of an organic dye-based recording material. As one proposal for solving this problem, it is contemplated to use carbon black which has good absorptivity over the entire wavelengths as a light absorbing material, but carbon black is inadequate to solve this problem because it becomes transparent to a light having a wavelength longer than its particle diameter.